We were youngsters in 1973 when Paul Simon recorded his song “American Tune,” too immature to understand the profoundity of his words, too busy listening to disposable records to grasp the greatness of his creation. The song has been covered many times subsequently, but now that Kurt Elling sings it on his new album “1619 Broadway: The Brill Building Project,” we finally get it. Simon’s song is a masterpiece. And Elling, accompanied only by his collaborator Laurence Hobgood on piano, sings masterfully, communicating the weariness and dreamy hope in Simon’s lyrics. “American Tune” is a work of art — the song and Elling’s interpretation.